1 Field of the Invention
The present invention relates to an image processor which processes digital image data for image reproduction.
2. Description of the Related Art
In a printer or the like wherein an image of full color is reproduced, digital image data R, G, B of three primary colors of red, green and blue of the additive color system which have been read by scanning a document are converted for image reproduction to data C, M, Y of three colors of cyan, magenta and yellow for color reproduction (of subtractive color system).
In the data processing of a full color image to convert the digital data of red, green and blue of the three primary colors to the data of the colors for image reproduction, it is demanded to reconcile the purity of black and the brightness of chromatic colors.
In the reproduction of black in a full color image, when black is reproduced by overlaying cyan, magenta and yellow, it is difficult to reproduce pure black due to the effects of the spectral characteristics of each toner. Then, the reproducibility of black is improved by using the subtractive mixing of the reproduction color data C, M and Y and black painting of black data K. That is, the reproduction color data C, M and Y are subtracted by a certain amount (under color remove amount), while the black data K is obtained from the reproduction color data C, M and Y to reproduce black by using black toner. However, in this method, though the purity of black is improved with increasing the degree of black painting, whereas the brightness of chromatic colors is deteriorated.
As explained above, the reproducibility of black and the improvement of the brightness of full color are not compatible as to the under color remove amount for chromatic colors and the amount of black paint for black. Therefore, it is necessary to control optimally the subtraction amount for chromatic colors and the amount of black paint for black.
It is better to emphasize an edge for an image such as a character. The read data is converted to the density data in order to adjust to the sense of naked eyes. When the edge emphasis is conducted on the data after the density conversion of the read data, the following problems arise: As to an edge portion of a character or a narrow line, the data changes more smoothly than before the density conversion, and sufficient edge emphasis is not performed. Further, the data change at the high density side is large so that an edge is easily detected at the high density side and even image noises are emphasized. This is ascribable to that the slope of the -log characteristic is small at the low density side and large at the high density side. As to a full color image, the hue changes if the edge emphasis is performed on the data after the color correction.